A disk drive is a common digital data storage device. For example, magnetic hard disk drives are typically used in connection with personal computers. FIG. 1a schematically illustrates an exemplary disk drive system 10, and FIG. 1b schematically illustrates a side view of the exemplary disk drive system 10 of FIG. 1a. Referring to FIGS. 1a and 1b, the system 10 includes a magnetic disk 14, and an actuator arm assembly 16 controlled by a control unit 22. A head 18 for reading data from and/or writing data to the disk surface may be affixed at one end of the actuator arm assembly 16. A plurality of concentric tracks may be laid on the surface of the disk 14 (an exemplary track 20 is illustrated in FIG. 1a), in which data may be written to and/or from which data may be read by the head 18. During operation, the disk 14 may be rotated at a substantially constant speed in, for example, direction 24. Although not illustrated in FIGS. 1a and 1b, the system 10 may include several other components known to those skilled in the art—e.g., a spin motor to rotate the disk 14, a read channel, a write channel, and so on.
When the disk 14 is not rotating (e.g., while not in operation) or rotating at a low speed (e.g., at a beginning or an end of operation), the head 18 may be in contact with the surface of the disk 14. However, during operation, the high speed of the rotating disk may cause air to flow under the actuator arm assembly 16 and head 18, and the resulting aerodynamic force may lift the head 18 off the surface of the disk 14, as illustrated in FIG. 1b (indicated by distance d). The distance d is commonly referred to as a flying height (FH).
In various embodiments, if the head 18 is located too high from the disk surface (i.e., high flying height), the magnetic fields produced by the head 18 may not be strong enough for accurate write operation, and/or the head 18 may not be able to accurately sense magnetic characteristics on the disk surface, thereby adversely affecting a write and/or read bit error rate. On the other hand, if the head 18 is located too close to the surface of the disk 14 (i.e., low flying height), the head 18 may accidentally touch or bump the surface of the disk 14, thereby damaging the disk 14 and/or head 18. Accordingly, accurate measurement and control of the flying height of a read/write head is generally desirable.
Several methods are currently available for measuring flying height of a head in a disk drive based on, for example, measurement of signal strength, cross coupling capacitance, read head temperature, harmonic ratio, etc., as is well known to those skilled in the art. For example, Jianfeng Xu et al. (“Head-Medium Spacing Measurement Using the Read-Back Signal”, IEEE Transaction on Magnetics, vol. 42, no. 10, pp 2486-2488, October 2006) and Brown et al. (U.S. Pat. No. 4,777,544, issued Oct. 11, 1988) disclose harmonic ratio flying height measurement based on a frequency ratio between two frequency domains (e.g., fundamental frequency and third harmonic frequency) of a predetermined repetitive readback voltage signal. More specifically, Jianfeng Xu et al. proposes an equation for flying height estimation, given by
                              d          =                      -                                                            3                  ⁢                  λ                                                  4                  ⁢                  λ                                            ⁡                              [                                                      ln                    ⁡                                          (                                                                        V                          3                                                                          V                          1                                                                    )                                                        +                  C                                ]                                                    ,                            Equation        ⁢                                  ⁢        1            where d is the estimated flying height, λ is a recording wavelength, C is a system constant, V1 and V3 are the amplitudes of the fundamental frequency and the third harmonic components, respectively, of the readback voltage signal.
The flying height measurement in Jianfeng Xu et al. and Brown et al. may be performed over a long period of time on one or more dedicated tracks (referred to herein as “flying height measurement tracks”). For example, FIG. 2 schematically illustrates an exemplary disk drive system 10a, including a disk 14 with dedicated flying height measurement tracks 40a and 40b. As illustrated in FIG. 2, the dedicated flying height measurement tracks 40a and 40b may be located at or near the inner most or outer most radius of the disk surface, although one or more flying height measurement tracks may also be imposed concentrically at any place in between the tracks 40a and 40b. Flying height measurement in Jianfeng Xu et al. and Brown et al. may be performed based on read channel circuit measurements of the fundamental and third harmonic frequencies of a read signal over the dedicated flying height measurement tracks 40a and 40b. The flying height measurement on the tracks 40a and 40b may then be assumed to be valid over all tracks and over all radial positions on the disk 14. Alternatively, the flying height of a head on a particular track (e.g., track 20) or at a particular radial position on the disk surface may be estimated by interpolating the flying height measurements of the dedicated flying height measurement tracks 40a and 40b. 
However, for various reasons (e.g., uneven disk surface, variation in disk speed, etc.), the flying height of the head may not be similar over the entire disk surface or over all the tracks. Also, for different radial positions on the disk, the linear velocity of the disk may be different, which may create different amount of aerodynamic lift of the head, thereby resulting in different flying heights of the head at different radial positions. Additionally, there may be a large time difference between two flying height measurements as determined from two different flying height measurement tracks—e.g., flying height measurement tracks 40a and 40b. 
Put differently, the existing flying height measurement techniques do not provide a method to measure an actual flying height at, for example, a specific track and/or at a specific radial position on the disk surface. In order to have a better control on the flying height, it may be desirable to measure the actual flying height value at any radial position and/or any track on the disk surface, rather than using approximate or interpolated flying height measurements from one or more dedicated flying height measurement tracks.
As is well known to those skilled in the art, a servo system in a hard disk drive, among other things, may enable a read/write head of the disk to follow a target track on the disk—i.e., maintain alignment of a reading or writing transducer with respect to a centerline of the target track. Several types of servo systems currently exist, including an embedded servo system that employs servo data on the same disk surface that stores user data. An embedded servo format for the disk surface may have a plurality of radially-extending servo-data regions (sometimes referred to as servo wedges) and an interspersed plurality of radially-extending user-data regions.
FIG. 3 schematically illustrates an exemplary disk 300. The disk 300 includes a disk surface 304 that is divided into a plurality of tracks in the form of concentric rings, e.g., tracks 308a, 308b, 308c, 308d, etc. In various embodiments, disk surface 304 may also be divided into a plurality of wedges, or sectors, to enable the data to be located circumferentially about the disk, e.g., data-storing sectors 312a, 312b, etc. In between adjacent data-storing sectors, there may be provided a servo wedge, e.g., servo wedge 316a between data-storing sectors 312a and 312b. The servo wedges may be used for storing servo positioning data to assist a servo circuit (not illustrated in FIG. 3) in ascertaining the current position of the head and/or aligning the head with a target track. It should be apparent that although the disk surface 304 is illustrated to have only four tracks, eight data-storing sectors and eight servo wedges (only a few of which are labeled in FIG. 3 for clarity), the disk surface 304 may have larger numbers of tracks, data-storing sectors and/or servo wedges. For example, the disk 300 may have 50, 200, 300, 600 or any other appropriate number of servo wedges.
Each of the data-storing sectors 312a, 312b, etc. may include a plurality of data tracks configured to store user data. For example, data-storing sector 312a may include data-tracks 324a, 324b, 324c, 324d, etc. (only one of the data-tracks, 324b, is illustrated for clarity) corresponding to tracks 308a, 308b, 308c, 308d, etc., respectively. Similarly, each of the servo wedges 316a, 316b, etc. may include a plurality of servo sectors, one for each track, configured to store servo positioning data. For example, servo wedge 316a may include servo sectors 320a, 320b, 320c, 320d, etc. corresponding to tracks 308a, 308b, 308c, 308d, etc. respectively. Thus, a servo sector of a track may be a portion of a servo wedge within the track, and may be located between data tracks in the same track. Accordingly, a single track may include a plurality of data tracks and a plurality of servo sectors.
In various embodiments, other formats of a disk may be possible. For example, although all tracks in FIG. 3 are illustrated to have the same number of data tracks and same number of servo sectors, in some embodiments, tracks that are radially inward towards the center of the disk may have a lower number of servo sectors and/or data tracks relative to those tracks that are radially further from the center of the disk.
FIG. 4 schematically illustrates a portion of an exemplary track 400 of a disk—e.g, disk 300 of FIG. 3. In various embodiments, track 400 may be any one of the plurality of concentric tracks in the disk 300 (e.g., track 308a of FIG. 3). Although track 400 may include a plurality of data tracks and a plurality of servo sectors, only portions of two data tracks, and a servo sector 420 embedded or sandwiched between the two data tracks are illustrated in FIG. 4.
As previously alluded to herein, the servo sector 420 may include one or more fields for storing servo information, which a disk head may use to synchronize and accurately position itself over the track 400 during a write and/or read operation performed on the track 400. The one or more fields of the servo sector 420 are discussed in greater detail below.
FIG. 5 schematically illustrates a section 500 of a disk surface—e.g., the disk surface 304 of disk 300 of FIG. 3. The section 500 may include a plurality of tracks, only four of which are illustrated in FIG. 5. The section 500 may also include servo wedge 520 (e.g., similar to the servo wedge 316a of FIG. 3) embedded between two data-storing sectors (e.g., similar to data-storing sectors 312a and 312b of FIG. 3). It will be apparent to those skilled in the art that only a portion of the servo wedge 520 is illustrated in FIG. 5 for purposes of clarity.
Each of the tracks illustrated in FIG. 5 may be similar to the track 400 of FIG. 4. Accordingly, referring to FIGS. 4 and 5, the servo sector 420 of FIG. 4 (or the servo wedge 520 of FIG. 5) may include a plurality of servo information fields, e.g., preamble field, servo sync mark (SSM) field, track/sector identification (ID) fields, one or more position error signal (PES) fields, and one or more repeatable run out (RRO) fields (RRO1 and RRO2, identified as R1 and R2 in FIG. 5), as is well known to those skilled in the art.
In various embodiments, the preamble field may comprise a periodic pattern which may allow a proper gain adjustment and/or timing synchronization of a read and/or write signal. The servo sync mark field may comprise special patterns for symbol synchronizing to a servo data. The track/sector ID field (identified as ID in FIG. 5) may include sector and/or track identification data of the respective servo sector, adjacent data tracks, and/or the track address. A servo control system (not illustrated in FIGS. 4 and 5) may process a read back signal of the preamble field, servo sync mark field, and/or the ID field to derive a coarse position of the head with respect to a target track.
In various embodiments, the position error signal fields may be used to more accurately align the head with the target track. The position error signal fields may include a plurality of servo positioning bursts (e.g., bursts A, B, C, and D, as illustrated in FIG. 5) for more accurate positioning of the head over the target track. As is well known to those skilled in the art, the centerline of some of the servo positioning bursts (e.g., burst A) may be aligned to the centerline of a corresponding data track, while the centerline of other servo positioning bursts (e.g., burst D) may not be aligned to the centerline of a corresponding data track. That is, one or more servo positioning bursts may have offsets with respect to a track centerline. The servo positioning bursts may be recorded at precise intervals, and fine head position control information may be derived from the servo positioning bursts for use in centerline tracking while writing data to and reading data from the target track.
In various embodiments, a repeatable runout may involve periodic deviations of the head, occurring with predictable regularity, from a target track caused by, for example, disk spindle motor runout, disk slippage, disk warping, vibrations, resonances, media defects, disk distortion due to clamping of the disk, electromagnetic imperfections due to low quality servo positioning bursts, etc. The RRO fields (e.g., R1 and R2, as illustrated in FIG. 5) may correct the head position to counter possible repeatable runouts of the head.
It will be appreciated by those skilled in the art that other servo information fields, although not illustrated in FIGS. 4 and 5, may also be present in a servo sector. In various other embodiments, one or more of the servo information fields (e.g., the RRO fields) may be absent from the servo sector of FIGS. 4 and 5.
The servo sector 420 and/or servo wedge 520 may be written on a disk during manufacturing of the disk. For example, in various embodiments, the preamble field, servo sync mark field, track/sector identification field, and/or one or position error signal fields (including the servo positioning bursts) may be written on the disk during manufacturing using a conventional stitch writing technique. Once these fields are written to the disk, calibration tests may be performed on the disk, based on which the RRO fields may be written on the disk.